Prefetching is a caching technique used to improve the performance of disk and memory systems. Whereas nominal caching increases performance by keeping copies of accessed data in the hope that it will be accessed again, prefetching loads the caching memory before an access to data in the hope it will be accessed soon. The information required for a successful prefetch includes:                What data to prefetch        The circumstances under which the prefetch should occur        The length of time to keep prefetched data cached (if no access occurs)        
The effectiveness of a disk prefetch is dependent on the correct prediction of the future read patterns over the disk. Predictions can be based on guesses or historical observation. An example of a typical guess involves the concept of “spacial locality” which predicts that a future read is likely to occur in proximity by address to the last read. Historical observations involve recognizing patterns of access, such as address B always follows A and address C always follows B.
If the wrong data is prefetched, no accesses to the data will occur and no performance improvements will be realized. Likewise, if the right data is fetched at the wrong time, it may be replaced by other caching data before the access occurs. Incorrectly specifying the “keep time” will have a similar effect.
In a storage system, defining a prefetch sequence and effectively guessing what future data accesses will be, is a computationally intensive and sometimes intractable task.